I completely forgot

I used past continuous tense, because it sounds like there was a plan for your sister to come to your apartment. This tense is for plans or activities that happen in the past and get interrupted. (Isn't English fun! : ) For example, "I was going to watch the movie, but I fell asleep." Simple past doesn't express the planned part. I added "past few" because it makes the time more clear. 

I completely forgot that my sister came was coming to my apartment as I have been busy these past few days.
I was going to cook pork soup for her but I couldn't. There was no pork or some vegetables in the fridge.
I just buy bought rolled Sushi and fried some sweet potato and soy beans., but she said that was delicious.